The present invention relates to fuser apparatus for electrostatographic printing machines and in particular to release agent management (RAM) systems for a heat and pressure roll fuser.
In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions or the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate.
To obviate the foregoing toner offset problem it has been common practice to utilize toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied to the fuser roll surface to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from adhering to the fuser roll surface.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
Various systems have been used to deliver release agent fluid to the fuser roll including the use of oil soaked rolls and wicks with and without supply sumps as well as oil impregnated webs. A another type of RAM system is disclosed in U.S. Pat. No. 4,214,549 granted to Rabin Moser on Jul. 29, 1980. As disclosed therein, release agent material is contained in a sump from which it is dispensed using a metering roll and a donor roll, the former of which contacts the release agent material and the latter of which contacts the surface of the heated fuser roll.
Xerox Disclosure Journal (XDJ) Volume 7, Number 3 dated May/June 1982 discloses a release agent management system for a roll fuser apparatus. The apparatus comprises a fuser roll to which silicone oil is applied in order to counteract toner offset to the fuser roll. The fuser roll cooperates with a softer pressure roll to fuse toner images to a copy substrate such as plain paper. The silicone oil which is contained in a sump is applied to the surface of the fuser roll by means of a rotating brush which is adapted to be rotated in the opposite direction to that of the fuser roll. The brush engages one end of a wick while the other end of the wick is immersed in the silicone oil. Thus, the brush picks up silicone oil from the wick and conveys it to the fuser roll surface. Since the brush rotates counter to the fuser roll the brush bristles strip the lead edge of the copy and deflect it down and away from the fuser roll. The brush fibers undergo a snapping or flicking action as they move out of the nip formed between them and the fuser roll. It is this action which yields the stripping action. The oil application rate is controlled by the brush fiber density and the velocity of the fuser roll.
In U.S. Pat. No. 5,200,786 granted to Fromm et al on Apr. 6, 1993, the donor roll of the '549 patent is replaced with a donor brush. As set forth in the '786 patent, the brush donor structure allows for the application of variable amounts of release agent material depending on the mode of operation. In other words, when color prints are being created a greater quantity of silicone oil is applied to the fuser roll compared to the amount applied when operating in the monochrome black mode.
In a donor brush RAM system of the type disclosed in the '786 patent, both fuser rolls (heated and backup) and the donor brush will reach an equilibrium with the oil on the metering roll when the oil is not removed from the fuser. The oil film thickness on the metering roll is much greater than the normal steady state oil film thickness on the brush and fuser rolls. If a machine were required to handle a single size substrate and the donor brush only applied oil to the fuser rolls in areas corresponding to that contacted by the single substrate the aforementioned equilibrium would not occur because the paper would continuously remove oil from the fuser roll. This would reduce the problem of excess release oil outside the paper path to only the narrow zone between the brush edge and paper path edge resulting from paper registration tolerance and subsystem tolerances. Excessive oil outside the paper path occurs when narrower width copy paper is used, particularly, when used for extended copy runs. However, other solutions for preventing excess oil accumulation outside the paper path have not been successful in solving the problem of excess oil occurring between copy sheets and during dead cycles.
Excess oil accumulation outside the paper path is a serious problem, particularly when long copy runs are made. Such oil accumulation must be prevented or at least minimized, otherwise oil will drip into the machine and/or cause copy quality defects from oil spots and/or steaks when switching to wider paper and/or cause deterioration of the fuser roll material when the outer surface of the fuser roll comprises silicone rubber. When roll deterioration occurs, a product of such deterioration is transferred to the wider paper. This results in the phenomena commonly referred to as red printout.